Well it depends on what you mean by last forever. I wouldn't consider until one partner dies to last forever because that is a mockery of the word forever as one partner lives as though they are no longer married when the declaration of the marriage was forever. If it is until both die, with the widower continuing to live their lives in commitment towards the other then that is beautiful and true I think, but still it is an imitation of forever.

I think both of these loves are possible and can be done through a mutual willingness to live for one another rather than themselves, especially in the latter case. When both partners are willing to let themselves live for the other that's a beautiful thing.

To make it last forever though, I think it involves something more radical, a change within the person that makes them see not only do they love this person, but they love the light of the Eternal within the person and can see God's Presence within the other person. If someone loves God and see's within the other person a spark of the Divine that causes them to have unconditional love that always says Yes to love, even onto forever, beyond the grave, so that any moment in the relationship whether the lowest or the highest you're willing to give a Yes to the relationship that you could live in that moment forever, that's how you truly make it last forever and in doing so divorce becomes an impossibility and makes the hard times and the good times equally beautiful not just as a learning experience, but in that unconditional Yes to the marriage.